Matrix metalloproteinases are thought to be involved in normal matrix turnover and repair, cell migration and metastasis, wound healing, and have been implicated in a wide range of processes involving connective tissue destruction. Initially, the tissue inhibitor of metalloproteinases (TIMP or TIMP-1) was the only inhibitor of its kind that was suspected to be involved in the regulation of matrix degrading metalloproteinases. We have recently published evidence for a family of TIMP-like inhibitors and one of those inhibitors, TIMP-2, has been purified by several groups and characterized as an inhibitor of the 72 kDa gelatinase, which is also known as Type-IV collagenase. This proteinase is suspected to play a critical role in cell migration and metastasis. All secreted mammalian metalloproteinases are secreted as inactive proenzymes. The models of activation that are bases on interstitial collagenase and stromelysin suggest that the proenzyme is inactive and stable and subsequently activated by proteolytic removal of the "propeptide". Preliminary data are presented to suggest that this model does not hold true for 72 kDa gelatinase. These studies show that the 72 kDa progelatinase is present in the culture medium of several cell types only as a complex with TIMP-2 and that this TIMP-2 molecule is not bound to the active site but rather bound to a second "stabilization site". Once TIMP-2 is removed from the stabilization site, the 72 kDa progelatinase autoactivates and generates active forms of the proteinase at 62 kDa and 42 kDa. The 42 kDa form is the most active form of the proteinase. Although the 72 kDa progelatinase: TIMP-2 complex can be activated by other means, only the less active 62 kDa form results. The TIMP-2 molecule also inhibits the active forms of the proteinase but does so by binding to a site that is different from the stabilization site. Furthermore, binding at either site is reversible yielding functional inhibitor. Because we have not been able to detect stable 72 kDa progelatinase not in complex with TIMP-2, we propose studies to determine when the complex is formed, before or after secretion. It is also proposed that both binding sites on the proteinase be identified and sequenced. Studies will also be directed at determining if TIMP-2 uses the same or two different sites to bind to the two sites on the proteinase. The proposed studies should generate data that will increase our understanding of the unusual interaction between a proteinase and its inhibitor that can also act as a stabilizer. The anticipated data should increase and likely alter our understanding of metalloproteinase activation and the regulation of matrix turnover.